Abstract

This article deals with sedimentary structures observed in shallow boreholes drilled during the geological survey of the area designated for the construction of expressway D11. Sediments of the Havlovice Member of the Trutnov Formation were found in boreholes drilled in the area near Nový Rokytník, where actually no larger outcrops exist. The investigation of these sedimentary structures makes easier to elucidate the paleoenvironment of the Trutnov Formation.The sedimentary structures provide evidence of fluvial environments of streams showing highly variable discharge, thus indicating their seasonal character. Flow fluctuation is evidenced, for instance, by structures with heterolithic stratification – flaser, wavy to lenticular bedding. The occasional occurrence of claystone beds outside the active flow and their erosion created a good prerequisite for the transport of claystone intraclasts by subsequent avulsion streams. Mud clasts accumulated on base levels or along foresets of cross lamination or cross-bedding of sandstones, and are thought to have been transported mostly by the traction mechanism over a short distance, judging from the dominance of their angular forms. Some more worn intraclasts might have undergone longer transport, and/or were part of a local seasonal turbulent flow. The turbulent flow, for example, at high-energy water flows creates deposits with well rounded slightly sorted intraclasts. Also, the abundant soft sediment deformation structures (SSDS) of the mixed sedimentary material demonstrate very rapid sedimentation in the environment of braided rivers. These SSDS indicate primary liquefaction and fluidization, caused by differential loading, inverted density gradients, and fast sediment deposition. They are developed as a result of deformation of the sediment during or soon after its deposition, or when compaction and/or diagenesis have started The studied boreholes provided load structures, convolute lamination and bedding, deformed cross-bedding, water escape structures, and sediment-injection structures.This area studied NNW of Nový Rokytník can be correlated with outcrops at the village of Starý Rokytník (an old quarry) south of the town of Trutnov.

References

Bhattacharya, A. (1997): On the origin of non-tidal flaser bedding in point bar deposits of the river Ajay, Bihar and West Bengal, NE India. - Sedimentology 44, 973-975.View article